Controlled area solder bonding for dies

ABSTRACT

A method of fabricating a semiconductor comprises forming a plurality of stud bumps in a pattern having a geometrical shape on a surface of a substrate, the pattern defining a periphery of a bonding area on the surface of the substrate, and placing a solder material in the bonding area such that the solder material is surrounded by the stud bumps. The solder material is heated to a temperature where the solder material begins to flow within the bonding area. A bonding surface of a die is pressed onto the stud bumps with a sufficient pressure to crush the stud bumps a predetermined extent such that the solder material substantially evenly spreads between the stud bumps within the bonding area. The solder material is then solidified to form a final solder area that conforms to the geometrical shape of the pattern of stud bumps.

BACKGROUND

Controlling the area where solder flows and limiting the bonding areaduring die attach is very difficult. Generally, the entire die isbonded, which results in excessive stress to the die. This can be verydetrimental to the performance of a die such as amicro-electro-mechanical systems (MEMS) sensor.

In general, a small solder area is all that is required to meet strengthrequirements for die attach. High temperature solders are usuallyextremely strong, and have the desirable attribute of high materialelasticity, which is critical to the predictable performance of MEMSsensors. When using solders for die attach, it can be difficult tocontrol the final area, shape, and location of the solder joint.

The most common solution for attaching die is to use a sufficient volumeof solder to completely cover the back side of the die, and to overflownext to the die. While this approach can make for a solid joint, it alsoimparts great stress on the attached die because of the mismatch of thecoefficient of thermal expansion (CTE) of the materials used, includingthe die material, the substrate or package to which the die is mounted,and the solder itself. For example, silicon die have a CTE in the rangeof about 2 to 3 ppm/° C. Ceramics to which die are often mounted, suchas alumina, have a higher CTE, typically around 7 ppm/° C., and printedcircuit boards have a much higher CTE.

Die stress can be reduced greatly by controlling the area over which adie is soldered. A small area near the center is usually best, but thiscan depend on the design and purpose of the die, especially when dealingwith MEMS sensors. Unfortunately, die solders are notoriously difficultto control as the solders wet unpredictably, and voiding and flow arehard to predict. Simply placing a small amount of solder near the centerof a die and reflowing can result in almost any shape and thickness offinal solder area, even when conditions are carefully controlled. Othermethods of die attach such as pure gold stud bump (GSB)thermocompression can control the bond area, but suffer from the soft,inelastic nature of the stud bump material.

SUMMARY

A method of fabricating a semiconductor comprises forming a plurality ofstud bumps in a pattern having a geometrical shape on a surface of asubstrate, the pattern defining a periphery of a bonding area on thesurface of the substrate, and placing a solder material in the bondingarea such that the solder material is surrounded by the stud bumps. Thesolder material is heated to a temperature where the solder materialbegins to flow within the bonding area. A bonding surface of a die ispressed onto the stud bumps with a sufficient pressure to crush the studbumps a predetermined extent such that the solder material substantiallyevenly spreads between the stud bumps within the bonding area. Thesolder material is then solidified to form a final solder area thatconforms to the geometrical shape of the pattern of stud bumps.

DRAWINGS

Understanding that the drawings depict only exemplary embodiments andare not therefore to be considered limiting in scope, the exemplaryembodiments will be described with additional specificity and detailthrough the use of the accompanying drawings, in which:

FIGS. 1A-1D are side views of a die attach process used in fabricating asemiconductor device according to one embodiment;

FIGS. 2A-2D are top views of the die attach process of FIGS. 1A-1D;

FIGS. 3A-3D are side views of a die attach process used in fabricating asemiconductor device according to another embodiment;

FIGS. 4A-4D are top views of the die attach process of FIGS. 3A-3D;

FIGS. 5A-5D are side views of a die attach process used in fabricating asemiconductor device according to a further embodiment;

FIGS. 6A-6D are top views of the die attach process of FIGS. 5A-5D;

FIGS. 7A-7D are side views of a die attach process used in fabricating asemiconductor device according to an alternative embodiment; and

FIGS. 8A-8D are top views of the die attach process of FIGS. 7A-7D.

In accordance with common practice, the various described features arenot drawn to scale but are drawn to emphasize specific features relevantto the exemplary embodiments.

DETAILED DESCRIPTION

In the following detailed description, embodiments are described insufficient detail to enable those skilled in the art to practice theinvention. It is to be understood that other embodiments may be utilizedwithout departing from the scope of the invention. The followingdetailed description is, therefore, not to be taken in a limiting sense.

A method is provided for controlling the solder bond area during dieattachment to a substrate in fabricating a semiconductor device. Thepresent method produces a controlled area solder bond on the surface ofthe die when attached to a substrate during packaging. The present dieattach process provides more stable stress on the die such as amicro-electro-mechanical systems (MEMS) die, and thus more stableperformance over temperature.

In the present technique, a plurality of stud bumps are formed in apredetermined pattern on a surface of a substrate, with the patterndefining a periphery of a bonding area on the surface of the substrate.The stud bumps form a “picket fence” around the periphery of the bondingarea on the substrate. A solder material is then placed in the bondingarea such that it is surrounded by the stud bumps. In exemplaryembodiments, the solder material can be in the form of a small solid“preform” such as a solder sphere, or small solder particles suspendedin a paste.

The solder material is heated to a temperature such that it begins toflow within the bonding area. A bonding surface of a die is then pressedonto the stud bumps with a sufficient pressure to crush the stud bumpssuch that the solder substantially evenly spreads between the stud bumpswithin the bonding area. The solder material is solidified to form afinal solder area that conforms to the shape of the predeterminedpattern of stud bumps.

The use of a “picket fence” of stud bumps placed near the desired edgeof the solder area does not necessarily form a dam for the solder, butgives the solder something to wet to. The surface tension of the moltensolder as it wets the stud bumps makes the solder cling to the studbumps and defines the final solder area. The stud bumps can also be usedto control the bondline thickness of the solder, since the stud bumpsresist force in a predictable fashion. Applying a certain force to thedie will crush the stud bumps to a predictable extent while spreadingthe solder to the stud bumps. If the pattern of stud bumps and forcesused to create the bond are carefully chosen, a joint with the strengthand elasticity of the high temperature solder can be formed withcontrolled location and size.

In a semiconductor device fabricated according to the present technique,the die is attached to the substrate at a controlled solder bond areathat includes the solder material that is substantially evenly spreadbetween the stud bumps coupled between the substrate and the die. Thestud bumps are coupled in a pattern having a geometrical shape anddefine a periphery of the solder bond area. The solder material conformsto the geometrical shape of the pattern of the stud bumps.

Additional details of the present approach are described as follows withreference to the drawings.

FIGS. 1A-1D and 2A-2D illustrate a die attach process according to oneembodiment. Initially, a plurality of stud bumps 110 are formed in apredetermined pattern having a geometrical shape around the edge of adesired bonding area 114 on a surface of a die attach substrate 118. Asshown in FIG. 2A, stud bumps 110 are formed in a circular pattern toform a “picket fence” on the surface of substrate 118, although othergeometric patterns may be utilized as described hereafter.

The stud bumps 110 can be formed on bonding area 114 by conventionaltechniques such as formation by a ball (wire) bonder. The stud bumps 110can be composed of various metals such as gold, platinum, copper,combinations thereof, or the like.

The substrate 118 can include, for example, a ceramic material such asalumina (aluminum oxide), aluminum nitride, silicon nitride, silicon,glasses; plated metals such as nickel alloys, including Kovar™ alloy(iron-nickel-cobalt), Alloy 42 (nickel-iron alloy), or the like; or aprinted circuit board (PCB).

A solder material in the form of a spherical solder preform 120, is thenplaced in bonding area 114 such that solder preform 120 is surrounded bystud bumps 110, as illustrated in FIGS. 1B and 2B. The solder preform120 is heated to the solder melting temperature so that it begins tolose its shape and flow within bonding area 114, as shown in FIGS. 1Cand 2C. The solder material can include, for example, eutectic alloyssuch as AuSn, AuSi, or AuGe, Indium alloys, or the like.

A die 130 is then pressed onto stud bumps 110 as depicted in FIG. 1D.The die 130 can include, for example, a MEMS sensor, a laser diode, aninertial sensor, or the like. Applying a certain force (such as a fewnewtons) to die 130 crushes stud bumps 110 a predictable extent whilespreading the solder material to stud bumps 110. The surface tension ofthe solder material makes it cling to stud bumps 110 as the soldermaterial wets stud bumps 110 while spreading. This defines a finalsolder area 132 in which the solder material is evenly spread betweenstud bumps 110. In this embodiment, the solder material conforms to thecircular pattern of stud bumps 110 to form a circular solder bond 134,as depicted in FIG. 2D.

FIGS. 3A-3D and 4A-4D illustrate a die attach process according toanother embodiment. Initially, a plurality of stud bumps 210 are formedin a predetermined pattern around the edge of a desired bonding area 214on a surface of a die attach substrate 218. As shown in FIG. 4A, studbumps 210 are formed in a circular pattern on the surface of substrate218. The stud bumps 210 can be formed on bonding area 214 byconventional deposition techniques, and can be composed of variousmetals as described previously.

A solder material in the form of a rectangular solder preform 220, isthen placed in bonding area 214 such that solder preform 220 issurrounded by stud bumps 210, as illustrated in FIGS. 3B and 4B. Thesolder preform 220 is then heated to the solder melting temperature sothat it begins to lose its shape and flow within bonding area 214, asshown in FIGS. 3C and 4C.

A die 230, such as a MEMS sensor die, is then pressed onto stud bumps210 as depicted in FIG. 3D. Applying a certain force to die 230 crushesstud bumps 210 while spreading the solder material towards stud bumps210. The surface tension of solder material 220 makes it cling to studbumps 210 as the solder material wets stud bumps 210 while spreading.This defines a final solder area 232 in which the solder material isevenly spread between stud bumps 210. In this embodiment, the soldermaterial conforms to the circular pattern of stud bumps 210 to form acircular solder bond 234, as depicted in FIG. 4D.

FIGS. 5A-5D and 6A-6D illustrate a die attach process according to afurther embodiment. A plurality of stud bumps 310 are formed in apredetermined pattern around the edge of a desired bonding area 314 on asurface of a die attach substrate 318. As shown in FIG. 6A, stud bumps310 are formed in a circular pattern on the surface of substrate 318.The stud bumps 310 can be formed on bonding area 314 by conventionaldeposition techniques, and can be composed of various metals asdescribed previously.

A solder preform 320 such as a cylindrical preform is located in bondingarea 314 such it is surrounded by stud bumps 310, as illustrated inFIGS. 5B and 6B. The solder preform 320 is then heated to the soldermelting temperature so that it begins to lose its shape and flow withinbonding area 314, as shown in FIGS. 5C and 6C.

A die 330, such as a MEMS sensor die, is then pressed onto stud bumps310 as depicted in FIG. 5D. Applying a certain force to die 330 crushesstud bumps 310 while spreading the solder material towards stud bumps310. The surface tension of the solder material allows it to cling tostud bumps 310 as the solder material wets stud bumps 310. This definesa final solder area 332 in which the solder material is evenly spreadbetween stud bumps 310. In this embodiment, the solder material conformsto the circular pattern of stud bumps 310 to form a circular solder bond334, as depicted in FIG. 6D.

FIGS. 7A-7D and 8A-8D illustrate a die attach process according to analternative embodiment. Initially, a plurality of stud bumps 410 areformed in a predetermined pattern around the edge of a desired bondingarea 414 on a surface of a die attach substrate 418. As shown in FIG.8A, stud bumps 410 are formed in a square or rectangular pattern on thesurface of substrate 418. The stud bumps 410 can be formed on bondingarea 414 by conventional deposition techniques, and can be composed ofvarious metals as described previously.

A solder preform 420 such as a cylindrical preform is then placed inbonding area 414 such that it is surrounded by stud bumps 410, asillustrated in FIGS. 8A and 8B. The preform 420 is then heated to thesolder melting temperature so that it begins to lose its shape and flowwithin bonding area 414, as shown in FIGS. 7C and 8C.

A die 430 such as a MEMS sensor die is then pressed onto stud bumps 410as depicted in FIG. 7D. Applying a force to die 430 crushes stud bumps410 while spreading solder material towards stud bumps 410. The surfacetension of the solder material makes it cling to stud bumps 410 as thesolder material wets stud bumps 410. This defines a final solder area432 in which the solder material is evenly spread between stud bumps410. In this embodiment, the solder material conforms to the square orrectangular shape of the pattern of stud bumps 410 to form a square orrectangular solder bond 434, as depicted in FIG. 8D.

Example Embodiments

Example 1 includes a method of fabricating a semiconductor device,comprising forming a plurality of stud bumps in a pattern having ageometrical shape on a surface of a substrate, the pattern defining aperiphery of a bonding area on the surface of the substrate; placing asolder material in the bonding area such that the solder material issurrounded by the stud bumps; heating the solder material to atemperature where the solder material begins to flow within the bondingarea; pressing a bonding surface of a die onto the stud bumps with asufficient pressure to crush the stud bumps a predetermined extent suchthat the solder material substantially evenly spreads between the studbumps within the bonding area; and solidifying the solder material toform a final solder area that conforms to the geometrical shape of thepattern of stud bumps.

Example 2 includes the method of Example 1, wherein the stud bumps areformed in a circular pattern on the surface of substrate.

Example 3 includes the method of Example 1, wherein the stud bumps areformed in a rectangular pattern on the surface of substrate.

Example 4 includes the method of any of Examples 1-3, wherein the studbumps comprise gold, platinum, copper, or combinations thereof.

Example 5 includes the method of any of Examples 1-4, wherein the soldermaterial is a spherical shaped preform.

Example 6 includes the method of any of Examples 1-4, wherein the soldermaterial is a rectangular shaped preform.

Example 7 includes the method of any of Examples 1-4, wherein the soldermaterial is a cylindrical shaped preform.

Example 8 includes the method of any of Examples 1-7, wherein the diecomprises a micro-electro-mechanical systems (MEMS) sensor, an inertialsensor, or a laser diode.

Example 9 includes the method of any of Examples 1-8, wherein thesubstrate comprises a ceramic material, a plated metal, or a printedcircuit board.

Example 10 includes the method of any of Examples 1-8, wherein thesubstrate comprises aluminum oxide, aluminum nitride, silicon nitride,silicon, or glass.

Example 11 includes a semiconductor device, comprising a substratehaving an upper surface; a die attached to the substrate at a controlledsolder bond area that includes a solder material that is substantiallyevenly spread between a plurality of stud bumps coupled between thesubstrate and the die; wherein the stud bumps are coupled in a patternhaving a geometrical shape and define a periphery of the solder bondarea; wherein the solder material conforms to the geometrical shape ofthe pattern of the stud bumps.

Example 12 includes the semiconductor device of Example 11, wherein thestud bumps are coupled in a circular pattern.

Example 13 includes the semiconductor device of Example 11, wherein thestud bumps are coupled in a rectangular pattern.

Example 14 includes the semiconductor device of any of Examples 11-13,wherein the stud bumps comprise gold, platinum, copper, or combinationsthereof.

Example 15 includes the semiconductor device of any of Examples 11-14,wherein the die comprises a MEMS sensor.

Example 16 includes the semiconductor device of any of Examples 11-15,wherein the die comprises an inertial sensor.

Example 17 includes the semiconductor device of any of Examples 11-14,wherein the die comprises a laser diode.

Example 18 includes the semiconductor device of any of Examples 11-17,wherein the substrate comprises a ceramic material, or a plated metal.

Example 19 includes the semiconductor device of any of Examples 11-17,wherein the substrate comprises aluminum oxide, aluminum nitride,silicon nitride, silicon, or glass.

Example 20 includes the semiconductor device of any of Examples 11-17,wherein the substrate comprises a printed circuit board.

Although specific embodiments have been illustrated and describedherein, it will be appreciated by those of ordinary skill in the artthat any arrangement, which is calculated to achieve the same purpose,may be substituted for the specific embodiments shown. Therefore, it ismanifestly intended that this invention be limited only by the claimsand the equivalents thereof.

What is claimed is:
 1. A method of fabricating a semiconductor device,comprising: forming a plurality of stud bumps in a pattern having ageometrical shape on a surface of a substrate, the pattern defining aperiphery of a bonding area on the surface of the substrate; placing asolder material in the bonding area such that the solder material issurrounded by the stud bumps; heating the solder material to atemperature where the solder material begins to flow within the bondingarea; pressing a bonding surface of a die onto the stud bumps with asufficient pressure to crush the stud bumps a predetermined extent suchthat the solder material substantially evenly spreads between the studbumps within the bonding area; and solidifying the solder material toform a final solder area that conforms to the geometrical shape of thepattern of stud bumps.
 2. The method of claim 1, wherein the stud bumpsare formed in a circular pattern on the surface of substrate.
 3. Themethod of claim 1, wherein the stud bumps are formed in a rectangularpattern on the surface of substrate.
 4. The method of claim 1, whereinthe stud bumps comprise gold, platinum, copper, or combinations thereof.5. The method of claim 1, wherein the solder material is a sphericalshaped preform.
 6. The method of claim 1, wherein the solder material isa rectangular shaped preform.
 7. The method of claim 1, wherein thesolder material is a cylindrical shaped preform.
 8. The method of claim1, wherein the die comprises a micro-electro-mechanical systems (MEMS)sensor, an inertial sensor, or a laser diode.
 9. The method of claim 1,wherein the substrate comprises a ceramic material, a plated metal, or aprinted circuit board.
 10. The method of claim 1, wherein the substratecomprises aluminum oxide, aluminum nitride, silicon nitride, silicon, orglass.
 11. A semiconductor device, comprising: a substrate having anupper surface; and a die attached to the substrate at a controlledsolder bond area that includes a solder material that is substantiallyevenly spread between a plurality of stud bumps coupled between thesubstrate and the die; wherein the stud bumps are coupled in a patternhaving a geometrical shape and define a periphery of the solder bondarea; wherein the solder material conforms to the geometrical shape ofthe pattern of the stud bumps.
 12. The semiconductor device of claim 11,wherein the stud bumps are coupled in a circular pattern.
 13. Thesemiconductor device of claim 11, wherein the stud bumps are coupled ina rectangular pattern.
 14. The semiconductor device of claim 11, whereinthe stud bumps comprise gold, platinum, copper, or combinations thereof.15. The semiconductor device of claim 11, wherein the die comprises aMEMS sensor.
 16. The semiconductor device of claim 11, wherein the diecomprises an inertial sensor.
 17. The semiconductor device of claim 11,wherein the die comprises a laser diode.
 18. The semiconductor device ofclaim 11, wherein the substrate comprises a ceramic material, or aplated metal.
 19. The semiconductor device of claim 11, wherein thesubstrate comprises aluminum oxide, aluminum nitride, silicon nitride,silicon, or glass.
 20. The semiconductor device of claim 11, wherein thesubstrate comprises a printed circuit board.